A San Francisco Fire fighter waits for water as he joins other departments from Daily City and San Bruno to fight a massive fire caused by an explosion in a San Bruno neighborhood Thursday, Sept. 9, 2010.

Photo: Lance Iversen, The Chronicle

A San Francisco Fire fighter waits for water as he joins other...

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Millbrae fire chief Dennis Haag at a press conference at the Bayhill shopping center in San Bruno, Calif., on Thursday, September 9, 2010.

Photo: Liz Hafalia, The Chronicle

Millbrae fire chief Dennis Haag at a press conference at the...

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Homes burn in a neighborhood off of Sneath Lane after an explosion, Sept. 9, 2010, in San Bruno, Calif.

Photo: Adm Golub, The Chronicle

Homes burn in a neighborhood off of Sneath Lane after an explosion,...

A "screwed-up" repair project at a Pacific Gas and Electric Co. control station in Milpitas caused pressure levels to rise on all the natural gas transmission lines serving the Peninsula just 50 minutes before the San Bruno pipeline exploded, newly released federal documents show.

The repair project set in motion a chain of unforeseen problems and blunders leading up to the Sept. 9 explosion that killed eight people and destroyed 38 homes, records released by the National Transportation Safety Board show.

Electricity was cut off to equipment that controls gas pressure in the Peninsula pipelines, two backup power supplies failed to work and critical communications between PG&E's gas control center in San Francisco and the Milpitas control station were lost, the records show.

When the power was cut to the control station in Milpitas, a system set up by PG&E automatically increased pressure on all three Peninsula pipelines, including the one coursing through San Bruno, according to PG&E employees interviewed by the safety board. The federal agency is investigating what caused the explosion.

Through it all, operators in the San Francisco control room proved powerless to fix the problem.

"We're screwed, we're screwed," one operator said minutes before the 30-inch gas transmission line exploded.

Company officials told investigators the pressure on the San Bruno line never exceeded the maximum safe operating level that PG&E established, although it came close to the limit. However, one pressure monitor indicated pressure went much higher than the safety limit.

PG&E has discounted the high reading as an anomaly caused by the repair job. Two experts interviewed by The Chronicle, however, said the higher reading shouldn't be discounted without more thorough investigation.

Unbeknownst to PG&E, the 1956 pipeline in San Bruno was vulnerable to rupture because of a poorly constructed weld on a longitudinal seam. The federal safety board's chairwoman has raised the possibility that PG&E set the line's maximum safe level too high because the company was unaware the pipe had such seams.

"No one yet knows what caused the tragic accident in San Bruno," PG&E spokesman Joe Molica in response to a Chronicle inquiry about the Milpitas incident. "PG&E's overriding objective from the outset of the investigation has been to help the NTSB identify the root cause of the accident."

PG&E's gas hub

The Milpitas terminal is a major intersection for the Bay Area's gas transmission system. Transmission pipelines enter the station from the east and south, and the three major lines that serve the Peninsula flow north from there. Gas operations are controlled by three to five operators in the San Francisco control room.

Hours before the San Bruno line exploded at 6:11 p.m. Sept. 9, two workers arrived at the Milpitas station to change out a faulty backup power system that supplies electricity to the pressure controls.

The pressure on the San Bruno line was running at 357 pounds per square inch. That was well below the maximum allowable operating pressure of 400 pounds, as well as the highest level at which PG&E generally ran the line, 375 pounds.

As one control room operator explained it afterward to his supervisor, the repair project quickly became "screwed up."

Cascade of problems

A second backup power supply system failed as the work was under way, records show. The Milpitas terminal supervisor told federal investigators that someone may have accidentally opened a circuit breaker at the station, cutting off another power source. And the repairmen in Milpitas had difficulties handling manual gas-control valves, according to workers' interviews with the safety board.

About 4:20 p.m., monitors in the San Francisco control room suddenly showed pressure dropping to zero on all three Peninsula lines. It was an erroneous reading caused when the repair project took the monitors off line, and it meant the control room had no idea what the lines' true pressures were, according to investigators' interviews with control room operators and a preliminary report by the federal safety board.

The pressure reading on all three Peninsula lines came back on at 4:32 p.m. and stayed between 358 and 360 pounds per square inch - comfortably within PG&E's safety level - for nearly an hour, records show.

A sudden jump

But at 5:21 p.m. - 50 minutes before the blast - the pressure on the lines jumped. On the San Bruno line, it hit 394 pounds within four minutes, records show.

Although still under the maximum allowable operating pressure, that was over the level at which the line usually ran. Control room operators told federal investigators they become concerned whenever any pipeline's pressure exceeds that level.

When the pressure in the Peninsula pipelines started rising, the bad news showed up on the computer screens in San Francisco.

"We're screwed. We're screwed," said one control room operator just 33 minutes before the explosion, after "high-high pressure alarms" started going off.

Just minutes before the disaster, control room operator Larry Roccholz told a co-worker, "We've got a major, major problem in Milpitas, and we've over-pressurized the whole Peninsula."

Pipeline pressure

A preliminary safety board report noted that valves at the Milpitas station that control pipeline pressure had defaulted to a wide-open position, as they were designed to do, when the electric power source to the control system was lost. The system is designed to cap pressure at 386 pounds per square inch.

PG&E officials have explained to investigators the default setting is necessary because a sudden decrease in gas pressure could be worse than an increase. It could cut off thousands of customers and create safety problems if, for example, people tried to relight their own pilot lights when the gas came back on, the company said.

Just before the San Bruno pipeline's seam ruptured, pressure on the line was 388 pounds, according to readings that PG&E provided to federal investigators. Oddly, however, another monitor reading taken just five minutes before the explosion registered the pressure on the San Bruno line at 623 pounds, records and interview transcripts show.

PG&E told the federal safety board that the higher reading was "affected" by the Milpitas repair work, indicating the utility believed it was inaccurate.

Couldn't be true

Mark Kazimursky, supervising engineer for the pressure control system, told safety board investigators that a reading that high was impossible because it exceeded the highest pressures in the incoming pipes supplying natural gas to the Milpitas terminal.

"We had some crazy pressures, indications that couldn't possibly be right," San Francisco control room operator Barry Mitchell told the safety board.

However, two experts said the high reading shouldn't be discounted so quickly.

Joe Weiss, a San Jose engineer who works on pipeline control systems, said the lower pressure of 394 pounds would not have caused the pipe to rupture and that the higher pressure reading was probably "rock solid."

"There's an awful lot of questions that have never been asked," Weiss said.

Donald Margolis, a mechanical engineering professor at UC Davis who specializes in control systems, said the opening and closing of valves caused by the repair project could have generated a "huge surge of pressure" much higher than the maximum safe operating pressure set by PG&E.

He suspects that pressure changes over the decades may have weakened the bad weld where the rupture finally happened, but that the pipe broke Sept. 9 because of a jolt.

"The fact that it broke that day, and not 10 years from now, is because of what was going on in the Milpitas station," Margolis said.

UC Davis chemical engineering Professor David Howitt said that if the San Bruno line had been well-built when it was put in place in 1956, the Sept. 9 pressure increases - at least the confirmed readings that topped out at 394 pounds - would probably have been harmless.

Howitt believes the pressure increases were the last in a long series of minor spikesthat wore out the bad seam weld over the decades.

"It's the straw that breaks the camel's back," Howitt said. "You never know when that's going to kick in. It is, after all, just a straw."